1. Field of Application
The invention relates to a vehicle navigation apparatus which estimates the current position of a vehicle by utilizing GPS (Global Positioning Satellite) position measurement data, for correction of relative position and travel direction information obtained by dead reckoning navigation calculations based on signals produced from sensors which are mounted on the vehicle.
2. Description of Prior Art
Types of vehicle navigation apparatus are now widely utilized, whereby the current position and travel path of the vehicle are indicated on a display device, superimposed upon a displayed road map of the region in which the vehicle is currently travelling. The apparatus may also be capable of determining and displaying the optimum route between that current position and a destination which is specified by the vehicle driver. With such an apparatus it is of course essential to determine the current position as accurately as possible under various different driving conditions. Typically with such an apparatus, an output signal produced from a gyroscope, indicative of changes in the vehicle course direction (i.e., detected as amounts of turning of the vehicle about a predetermined axis of the gyroscope) is used, with each such change representing a change in the travel direction of the vehicle in relation to a previously determined absolute travel direction (where, for brevity of description, the term xe2x80x9ctravel directionxe2x80x9d is used herein to refer to an estimated instantaneous direction of travel of a vehicle, derived together with an estimated current position of the vehicle). The direction change information from the gyroscope is used in conjunction with distance information expressing a distance that has been traveled by the vehicle relative to some preceding (absolute) estimated position of the vehicle, i.e., distance information obtained based on an output signal from a vehicle speed sensor, to perform dead reckoning calculations to obtain the estimated current position and travel direction of the vehicle. Such methods are described for example in Japanese patent HEI 8-54248, etc.
With such a method, on-board sensors of the vehicle itself are used to detect the vehicle position and travel direction, so that there is the disadvantage that it is not possible to obtain absolute position values.
Furthermore, when a gyroscope is utilized to measure changes in vehicle travel direction, the measurement is based on detecting values of angular velocity of rotation about the aforementioned predetermined axis of the gyroscope. When that axis does not correspond to the axis about which the vehicle actually rotates when performing a turn, then the conversion gain of the gyroscope (which is a proportionality constant, predetermined beforehand as a conversion factor for conversion to angular velocity) will differ from the correct conversion factor. Thus a conversion gain error (referred to in the following simply as the gain error) will arise. Moreover when a gyroscope is utilized, the value of output voltage of the signal produced from the gyroscope when the detected angular velocity is 0 (deg/s) is used as a reference voltage value, with that value being referred to in the following as the offset. However this offset may vary, due to various factors, so that when actual angular velocity is 0 (deg/s), the difference between the output voltage from the gyroscope and the offset may not be zero. As a result, a drift error will arise the in angular velocity detection results. Due to the above reasons, errors may arise in determining an amount of change in the vehicle travel direction, and the accuracy of detecting the current position of the vehicle will thereby be reduced. The effects of such drift and offset errors will be cumulative.
Because of the sources of inaccuracy described above, methods of vehicle position detection are utilized whereby position measurement data conveyed by radio waves transmitted from a source such as the GPS system are used to periodically obtain absolute position and travel direction information, for use in correcting the positions and directions that are derived by dead reckoning based on the on-board sensor outputs as described above. Specifically, while the vehicle is being driven in a condition in which such radio waves conveying the GPS position measurement data can be received, successive absolute positions and corresponding travel directions of the vehicle are derived based on these data at periodic intervals, with each set of information thus obtained being used to correct the positions and travel directions which are derived by dead reckoning using the vehicle on-board sensors.
However with the GPS system (as available for public use), the absolute position and direction estimates which are obtained thereby contain substantial amounts of randomly varying error, which may have a magnitude of up to approximately 100 meters in the case of position estimates. The successive relative position and travel direction estimates which are obtained from the dead reckoning calculations on the other hand will contain relatively small amounts of such randomly varying error, under a normal condition of operation of the vehicle, but may contain significant amounts of drift and offset error, for the reasons mentioned above.
For that reason, position and travel direction information derived based on the GPS data are generally subjected to a form of filter processing for reducing the effects of the random errors in these data, with the result being applied to correct the estimated positions and travel directions derived by dead reckoning. The most widely utilized form of such processing is the Kalman filter.
Successive sets of corrected vehicle position and travel direction estimates which are thereby derived are combined to obtain an estimate of the path which has been traveled by the vehicle up to the current position. Periodically, that estimated travel path is applied in map matching processing, i.e., the path is compared with data expressing a road map of a region in which the vehicle is currently travelling, to make use of the fact that the vehicle location is in general constrained to streets or freeways, etc., and thereby further increase the accuracy of a finally estimated current position of the vehicle. In that way it becomes possible to accurately obtain and display the route which is being traversed by the vehicle and its current position.
Under normal driving conditions, such a type of vehicle navigation apparatus can provide accurate results. However substantial amounts of error may arise in the estimated vehicle travel direction that is derived by such an apparatus, under some special circumstances. A first case of such special circumstances is when the vehicle is driven into a location such as a multistory car park, then performs a number of successive turns during a short time interval, e.g., while driving up and around a series of ramps within the car park. This can result in substantial error for the following reasons:
(a) The gain error of the gyroscope will cumulatively increase as the successive turns are executed, even if the detection axis of the gyroscope is correctly oriented with the turning axis of the vehicle and the gyroscope is mounted correctly in relation to the direction of the action of gravity under normal driving conditions. Specifically, when driven up the ramps within a multistory car park, and thereby performing successive turns, the attitude of the vehicle will be altered due to vehicle roll to the left or right side as the successive turns are executed, and due to the lateral axis of the vehicle becoming tilted (i.e., occurrence of pitch) by the slope of the ramps. Such changes in the vehicle attitude may cause changes in the orientation of the detection axis of the gyroscope in relation to the direction of action of gravity. As a result, the conversion factor of the gyroscope will change, so that the conversion gain will contain a gain error. Furthermore, errors due to variations in offset will cumulatively increase, when the vehicle executes such repetitive turns, so that this error will also become large. As a result, after completing such a succession of turns within a multistory car park, the estimated vehicle direction that is then obtained by dead reckoning calculation using the gyroscope may contain a large amount of error.
(b) In general, it will be difficult or impossible to receive GPS radio waves when the vehicle is located within such a multistory car park, so that it will not normally be possible for the vehicle navigation apparatus to effect correction of the estimated travel direction based on GPS-derived data.
(c) Even if acquisition of GPS position measurement data is possible under such a condition (e.g., immediately following completion of such a succession of turns) it will in general not be possible to apply appropriate map matching processing to the results, (since the vehicle is not located on a street or highway), so that the GPS position measurement data cannot be used in accurately correcting the error which arises in the estimated travel direction.
A second of such special conditions is when the vehicle is driven onto a turntable and is then turned through some arbitrary angle, with the vehicle engine ignition switched off so that the vehicle navigation apparatus is inoperative. When the ignition is subsequently switched on and the vehicle thereafter starts to be driven, there may be a large amount of deviation between the direction of travel that is indicated by the vehicle navigation apparatus at that time (which is the direction of travel most recently estimated prior to switching off the ignition and thereby cutting off the operating power for the apparatus) and the actual direction of travel.
When such a special condition has occurred and the vehicle then begins to be driven (e.g., out of a parking lot), a certain amount of time must elapse before sufficient correction of the direction error can be achieved. The reason for this is that, as described above, each vehicle position and travel direction which is derived from received GPS positioning data is not applied directly, but instead is utilized in processing such as Kalman filtering, with the results being then applied to correct the dead reckoning estimates of position and travel direction.
So long as the travel direction estimates which are derived from the dead reckoning calculations successively vary in a normal manner, no problems will arise. However when a sudden abnormal change occurs in that series of travel direction estimates, for a reason such as described above, then even if acquisition of the GPS positioning data can be achieved immediately after the vehicle begins to be driven thereafter, a significant amount of time will elapse before the position and travel direction estimates that are derived from the filter processing will be of sufficient accuracy to enable successful map matching processing to be performed. That is to say, a number of GPS positioning data acquisition operations must be successively performed, before effective correction of the positions and travel direction derived by the vehicle navigation apparatus for use in map matching processing become sufficiently accurate for that purpose.
As a result, when the vehicle starts to be driven after such a special condition has occurred, a certain amount of delay may occur before the vehicle navigation apparatus begins to generate an accurate display of the vehicle current position and travel path, i.e., a delay during which there may be a large amount of error in the information displayed by the vehicle navigation apparatus.
It is an objective of the present invention to overcome the disadvantages of prior types of vehicle navigation apparatus as set out above, by providing a vehicle navigation apparatus whereby even when a vehicle has been operated under special circumstances whereby a large amount of error in the estimated travel direction of the vehicle has arisen, correction of the error can be rapidly performed.
To achieve this objective with a vehicle navigation apparatus according to the present invention, when such a special condition has occurred and the vehicle thereafter begins to be driven, then after GPS position measurement start to be acquired, each of respective estimated vehicle positions which are derived directly based on the GPS data, without application of filter processing, is compared with a corresponding (i.e., substantially concurrently derived) vehicle position that is obtained by dead reckoning calculations based on the vehicle sensor outputs. If there is more than a predetermined degree of deviation between these two positions, then the vehicle position derived from the GPS positioning data is applied directly to correct the corresponding vehicle position obtained from the dead reckoning calculations, while the travel direction derived from the GPS data is similarly directly applied. This operation is repeated for successively acquired GPS-derived position estimates, until it is judged that the aforementioned deviation magnitude is no longer exceeded. Thereafter, normal operation is commenced, with filter processing such as Kalman filter processing being applied to the GPS-derived position and travel direction data and the results applied to correct the corresponding estimated positions and travel directions which are derived from the dead reckoning calculations.
It can be understood that with such a form of operation of a vehicle navigation apparatus, although it is known that a certain degree of error exists in the vehicle positions which are derived from the GPS position measurement data when utilized directly without filter processing, these data are used directly in order to achieve more rapid estimation of the actual travel path of the vehicle, after a special condition has occurred whereby there is a large amount of error in the vehicle travel direction as estimated by dead reckoning calculations.
In that way, when such a condition of substantial error in the estimated travel direction has arisen, accurate position and travel path information can quickly begin to be obtained and displayed by the vehicle navigation apparatus.
The vehicle navigation apparatus may be configured with means for judging that the distance between a vehicle position as estimated using the on-board sensors for dead reckoning calculation and the corresponding vehicle position as derived directly from GPS position measurement data exceeds a predetermined amount, and for directly applying the position and travel direction estimates obtained directly from the GPS measurement data to correct the corresponding dead reckoning position and travel direction estimates when it is found that the predetermined amount is exceeded.
Alternatively, the vehicle navigation apparatus includes means for registering the estimated position of the vehicle at the time when such a special condition may have arisen, as a reference position. This reference position may be the latest position that was estimated by the vehicle navigation apparatus prior to switching off the vehicle ignition, and thereby disconnecting the power supply to the vehicle navigation apparatus. Alternatively, or in addition to such means, the vehicle navigation apparatus can include means for detecting that the vehicle has been driven in succession around a number of complete Turns within a short time interval, and for establishing the estimated position of the vehicle upon completion of these turns as being the reference position. In that case, the vehicle navigation apparatus may further include means operating (each time that a GPS data acquisition operation is executed) for calculating a vector extending from the reference position to a vehicle position that is currently estimated by the vehicle navigation apparatus based on dead reckoning and a vector extending from the reference position to the corresponding vehicle position that is derived directly from the GPS position measurement data (with no filter processing applied), and for judging whether the angle between these vectors exceeds a predetermined value. If that value is exceeded, then the GPS position measurement data are used directly, without filtering, to correct the vehicle position and travel direction which are obtained by dead reckoning. If the angle between the vectors does not exceed the predetermined value, then the position and travel direction information obtained based on the GPS position measurement data are applied in the usual manner, through filter processing. Hence, the invention enables a vehicle navigation apparatus to be provided whereby, even when some special circumstance has arisen which has caused the travel direction derived by the apparatus through dead reckoning calculations to become substantially different from the actual travel direction, GPS position measurement data are utilized in a manner whereby the actual travel path and current position of the vehicle can be rapidly estimated by the vehicle navigation apparatus, to a sufficient degree of accuracy for use in map matching processing.